# Phase 21 — Windowed Streaming Buffer (bounded client memory for long streams)

Product spec. Status: **design / framing — implementation-ready pending Daniel's open-question calls.**
Author: product-designer. Date: 2026-06-23. **No code has been written by this doc.**
Surface: **public listener site only** (`DeepDrftPublic.Client` player stack + `DeepDrftPublic`
TypeScript audio interop). No CMS (`DeepDrftManager`) change. No data-model or schema change. The one
server touch is **reuse, not new surface**: the existing `DeepDrftAPI` HTTP `Range: bytes=X-`
partial-content primitive (Phase 4, landed) is the load-bearing dependency; this phase adds no new API
endpoint.

> **Sequencing dependency (Daniel, 2026-06-23): Phase 18 (Opus Low-Data Streaming) comes BEFORE this
> phase.** Format support — specifically the derived **Ogg Opus fullband 320** low-data delivery path
> (`product-notes/phase-18-opus-low-data-streaming.md`) — is a prerequisite that sequences ahead of
> windowing. Phase 21's windowing must work across **both** delivery formats (lossless WAV and Opus).
> Its C5 invariant below already anticipated this ("must not foreclose MP3/FLAC"); **Opus is now the
> concrete VBR/containerized driver of C5.** Windowing an Opus stream uses the decoder's **accurate
> index-based** byte↔time mapping (`OpusFormatDecoder.calculateByteOffset` — a binary search in the Phase 18
> precomputed seek index), exactly the C5 case — *not* the exact CBR-WAV `byteRate` math, and *not*
> approximate Ogg-page interpolation. **Correction (Daniel, 2026-06-23):** an earlier draft described the
> Opus mapping as "approximate page interpolation"; the Phase 18 seek-model resolution rejected that — Opus
> seeking is **accurate**, backed by a precomputed seek index built at transcode time, so refill resolves to
> the *exact* page offset. The windowed refill controller calls the **same** index resolver an explicit seek
> does (Phase 18 §3.4a D); a window opening away from byte 0 still decodes via the Phase 18 sidecar setup
> header. Build the window machinery format-agnostically (§2 C3/C5) so it inherits Opus for free.

---

## 1. Goal

Bound the **client memory** a playing track consumes to a small, configurable forward window —
**independent of total stream length** — so a 1 GB+ DJ MIX (Phase 9 `Mix` medium: a single long track)
plays without the whole decoded PCM accumulating in the browser.

**The defect, stated precisely.** The network path already streams in adaptive 16–64 KB chunks
(`StreamingAudioPlayerService.StreamAudioWithEarlyPlayback`) — that part is fine. The accumulation is on
the **decode side**: `PlaybackScheduler` holds `private buffers: AudioBuffer[]` and **never evicts**
("Supports pause/resume/seek by **retaining all buffers**" — its own doc comment). Every 64 KB segment
the `StreamDecoder` decodes is pushed via `addBuffer()` and kept for the life of the track. Decoded PCM
is **larger than the compressed-or-raw source** in memory (Web Audio `AudioBuffer` is 32-bit float per
sample per channel — a 16-bit stereo WAV roughly **doubles** in size once decoded), so a 1 GB WAV becomes
~2 GB of retained `AudioBuffer` float data. That is the OOM.

**One-line framing:** today the player decodes the whole track into memory and keeps it; Phase 21 makes
it keep only a sliding forward window and discard what has already played, refilling on demand from the
Range primitive it already uses for seek.

---

## 2. Constraints / invariants (the contract that must hold)

These are non-negotiable. The §3.5 streaming seam (root `CLAUDE.md` "Streaming-first audio playback";
`CONTEXT.md §3.5`) is called *the most architecturally load-bearing part of the playback path* by both
docs. This phase **modifies that seam** — so the contract it must preserve is spelled out here.

- **C1 — The seek-beyond-buffer Range path is the substrate, kept intact.** Phase 4 landed HTTP
  `Range: bytes={offset}-` → `206 Partial Content` end to end (client `TrackMediaClient` →
  `DeepDrftPublic` proxy → `DeepDrftAPI`), and `StreamDecoder.reinitializeForRangeContinuation` retains
  the parsed format header on a continuation body (no re-parse). Windowed refill is a **generalization of
  this exact path** (§3.1) — it must not require a second, divergent fetch mechanism.
- **C2 — Playback start latency unchanged.** Today playback starts as soon as a configurable minimum
  buffer count is queued (header-derived duration, not full-file). The window model must keep first-audio
  latency at parity — bounding memory must not reintroduce a fetch-then-play stall.
- **C3 — The format-decoder abstraction is untouched.** `IFormatDecoder` owns all format-specific
  byte math; `AudioPlayer.createFormatDecoder` already dispatches on `Content-Type` (WAV/MP3/FLAC
  decoders all wired today — verified 2026-06-23; an `OpusFormatDecoder` joins them in Phase 18).
  Windowing lives in the
  **format-agnostic** layer (`PlaybackScheduler` eviction + `StreamDecoder`/player refill
  orchestration); it must add **no** format-specific branches. A future wired MP3/FLAC decoder inherits
  windowing for free.
- **C4 — Read-only playback only.** This is a memory-management change, not a UX change. No new
  user-visible control, no change to seek/transport semantics beyond what the listener already
  experiences. Seek must still feel identical.
- **C5 — Must window both delivery formats (WAV lossless AND Opus low-data).** Byte↔time mapping for
  refill is exact and cheap for WAV (CBR: `byteRate` from the header). **Phase 18 (Opus) is sequenced
  before this phase and is the concrete VBR driver here** — and its mapping is **also exact**, but by a
  different mechanism: an Ogg Opus 320 stream has no linear time↔byte relationship, so
  `OpusFormatDecoder.calculateByteOffset` resolves via a **precomputed seek index** (granule→byte, built at
  transcode; Phase 18 §3.4a), a binary search that returns the exact page offset — **not** an approximate
  page interpolation. (An earlier draft of this invariant said "approximate"; the Phase 18 seek-model
  resolution, Daniel 2026-06-23, made Opus seeking accurate. Corrected here.) The window machinery must
  express refill purely in terms of the decoder's existing `calculateByteOffset`, so the same code windows
  WAV (via `byteRate`) and Opus (via the index) — **no WAV-special-cased offset math in the window layer**,
  and no approximation for either. A window that opens away from byte 0 must also prepend the decoder's
  retained/sidecar setup header (Phase 18 §3.4a B) — the format-agnostic refill path already routes
  continuations through the decoder's header-carry, so this comes for free. (MP3/FLAC decoders are already
  wired in the registry too — the registry dispatches on content-type today; an `OpusFormatDecoder` joins
  them in Phase 18.)
- **C6 — No regression to the single-instance JS decoder concurrency guarantees.** The current code is
  careful that only one streaming loop touches the single JS `StreamDecoder` at a time
  (`DrainActiveStreamingTaskAsync`, the `_streamingCancellation` identity dance). Windowed refill
  introduces *more* mid-stream fetches; it must route through the **same** drain/cancellation discipline,
  not around it.
- **C7 — The Mix visualizer's data source is independent and must stay that way.** The Phase 10/12
  WebGL2 lava visualizer renders from a **preprocessed high-res waveform datum** fetched per-track
  (`GET api/track/{entryKey}/waveform/high-res`), **not** from live decoded PCM. Confirmed: evicting
  played `AudioBuffer`s cannot starve the visualizer — it never read them. The window model is invisible
  to the visualizer. (This is the canonical 1 GB case *and* the case that proves the eviction is safe.)

---

## 3. Architectural shape

### 3.0 The mental model

A track's audio is a byte range `[0, fileLength)` on disk. At any moment the listener is at playback
position `P` (seconds → byte offset via the format decoder). The player should hold decoded
`AudioBuffer`s only for a bounded window roughly `[P - back, P + ahead]`:

- **forward fill (`ahead`)** — enough decoded lookahead that playback never starves (covers the existing
  500 ms scheduler lookahead plus network jitter headroom);
- **back-retain (`back`)** — a small amount of *already-played* audio kept so a short seek-back does not
  trigger a network refetch;
- **evict** — anything older than `P - back` is dropped (`AudioBuffer` references released → GC reclaims
  the float data);
- **refill** — when forward decoded lookahead drops below a low-water mark, fetch+decode more from the
  current byte position; when the window's tail is evicted and the listener seeks back past it, refetch
  that region via the Range primitive (the seek-beyond-buffer path, run *backwards*).

This is a **ring/sliding-window buffer keyed on playback position**, driven by high/low-water marks —
the standard bounded-producer/bounded-consumer pattern, transplanted onto the decode→schedule seam.

### 3.1 Why this is a generalization of seek-beyond-buffer, not a new mechanism

The seek-beyond-buffer path already does **every primitive** the window needs, just triggered manually
and one-shot:

| Window operation              | Existing seek-beyond-buffer machinery it reuses                                    |
|-------------------------------|-----------------------------------------------------------------------------------|
| Discard buffers, keep offset  | `PlaybackScheduler.clearForSeek()` + `setPlaybackOffset()` (clears buffers, retains the absolute-time anchor) |
| Fetch from a byte offset      | `TrackMediaClient.GetTrackMedia(key, byteOffset)` → `Range: bytes=X-` → 206       |
| Decode a header-less body     | `StreamDecoder.reinitializeForRangeContinuation(remainingByteLength)`              |
| Map time → byte offset        | `StreamDecoder.calculateByteOffset()` → `IFormatDecoder.calculateByteOffset()`    |
| Single-loop safety on refetch | `_streamingCancellation` swap + `DrainActiveStreamingTaskAsync()`                  |

The difference is **eviction does not exist yet** (the scheduler only ever `clear()`s wholesale) and
**refill is one-shot** (a seek, not a continuous low-water-triggered loop). So the new work is two
seams: a *partial-evict* on the scheduler, and a *position-driven refill controller* on the player. The
fetch/decode/offset plumbing is reused verbatim.

### 3.2 The three candidate directions

Per file convention the alternatives are recorded; the recommendation follows.

**Direction A — Sliding window on the existing single forward stream (recommended).**
Keep the current model where the C# loop reads one forward HTTP stream and pumps chunks into the JS
decoder. Add two things: (1) `PlaybackScheduler` gains *partial eviction* — drop buffers whose
absolute-time end is older than `P - back`, adjusting its index bookkeeping so `getCurrentPosition()`
and scheduling stay correct against a buffer array that no longer starts at index 0; (2) a
*back-pressure* signal — when forward decoded lookahead exceeds the high-water mark, the C# loop
**pauses reading** the HTTP stream (stops calling `ReadAsync`) until playback drains it below low-water,
then resumes. Memory is bounded by high-water + back-retain. Seek-back beyond the retained window falls
through to the **existing** seek-beyond-buffer path unchanged.
*Why recommended:* smallest change to the load-bearing seam; reuses the live forward stream (no extra
connections in the common case); eviction and back-pressure are the only genuinely new mechanisms, and
both are local (one to the scheduler, one to the read loop). Back-pressure via "stop reading the socket"
is exactly how TCP flow control already wants to behave — pausing `ReadAsync` lets the kernel window
close; we are not fighting the transport.

**Direction B — Discrete window segments, each its own Range fetch.**
Treat the file as fixed-size byte segments (e.g. 4 MB). Hold N decoded segments around `P`; fetch the
next/previous segment via a fresh Range request as the window slides; discard the far segment. No live
long-lived forward stream — every window is an independent 206.
*Why not (default):* turns one connection into many short Range requests (more proxy hops through
`DeepDrftPublic`, more server-side `WavOffsetService`-style header synthesis, more places a fetch can
fail mid-stream — worsening the §1.6 error surface), and the byte↔time segment math must be exact at
every boundary. It *is* the cleaner model for true random-access (and the better base if seeking-heavy
usage dominates), so keep it as the fallback if Direction A's back-pressure proves leaky in practice.
Borrowed prior art: HLS/DASH segment windows and the MSE `SourceBuffer.remove()` eviction model — this
is how every production HTML5 adaptive player bounds memory. We are doing the hand-rolled equivalent
because the stack is a bespoke Web Audio graph, not `<media>` + MSE.

**Direction C — Adopt MediaSource Extensions (MSE) and let the browser manage the buffer.**
Stop hand-rolling the decode→schedule graph for long tracks; feed the Range stream into a `SourceBuffer`
and let the browser evict via its built-in quota + `remove()`. Memory management becomes the platform's
problem.
*Why not — RESOLVED, rejected (Daniel, 2026-06-23; see OQ5):* MSE does not accept raw WAV/PCM — it
wants containerized formats (fragmented MP4/WebM, or MP3/AAC elementary streams). The entire bespoke
visualizer/spectrum graph is wired to the Web Audio `AudioContext`, not a `<media>` element. Adopting
MSE is a **rewrite of the playback substrate**, not a windowing change. It *looked* like the real
long-term answer once compressed delivery arrived — but Daniel has decided compressed delivery
(**Phase 18 Opus**) will feed the **same bespoke graph** via the `IFormatDecoder` seam, so the
compressed-delivery move that would have justified MSE happens *without* surrendering the graph. **The
bespoke graph is a deliberate long-term commitment; MSE is rejected.** Direction A is therefore the
permanent destination, not a stopgap that MSE will retire. Recorded as considered-and-declined.

### 3.3 Recommended direction: A, with B held as the documented fallback

Direction A is the smallest coherent change that hits the headline (bounded memory under a 1 GB stream)
while honoring C1–C7. It keeps the live forward stream, reuses the seek-beyond-buffer path for the only
genuinely random-access case (seek-back past the retained tail), and isolates the two new mechanisms.
**The final architecture and the exact eviction/back-pressure API are staff-engineer's call at
implementation** (per file convention); this spec fixes the *shape* and the invariants, not the method
signatures.

### 3.4 SOLID / road-not-taken rationale

- **SRP, preserved.** Eviction is a `PlaybackScheduler` concern (it already owns buffer storage); refill
  orchestration is a player-service/`StreamDecoder` concern (they already own the fetch loop); byte↔time
  math stays in `IFormatDecoder`. No responsibility crosses a boundary it does not already own.
- **OCP, via C3/C5.** Windowing added in the format-agnostic layer means wiring MP3/FLAC later changes
  zero window code. The window expresses refill through `calculateByteOffset` — the one seam the
  decoders already implement.
- **The seam stays single-writer (C6).** Every new refetch routes through the existing
  cancellation/drain discipline, so "only one loop touches the JS decoder" remains true. This is the
  rule most likely to be violated by a naive implementation and is called out as a hard invariant.
- **Road not taken — eager full decode with a memory cap that just stops decoding.** Tempting (decode
  until you hit a byte budget, then stop) but it breaks playback of long tracks past the cap entirely —
  it bounds memory by *refusing to play the rest*, not by sliding. Rejected: it is a degradation, not a
  feature.

---

## 4. Use cases

- **UC1 — Play a 1 GB+ DJ MIX start to finish (the headline).** Memory stays bounded throughout; the
  listener experiences continuous playback identical to a short track.
- **UC2 — Seek forward within a long track.** Already handled by seek-beyond-buffer; under windowing the
  forward seek clears the window and refills at the target — no behavior change, now with eviction so the
  pre-seek region does not linger.
- **UC3 — Seek back a few seconds.** Served from the back-retain window with **no** network refetch
  (the reason `back` exists).
- **UC4 — Seek back far, past the evicted tail.** Falls through to the existing seek-beyond-buffer Range
  fetch, run toward an earlier offset. (Open question OQ2 — see §6.)
- **UC5 — Pause a long track for a long time.** Memory stays at the bounded window size while paused (no
  continued decode). On resume, forward fill restarts from the low-water trigger.
- **UC6 — Mix detail page with the lava visualizer running.** Visualizer reads its preprocessed datum
  (C7); windowing is invisible to it. Confirmed non-interaction.

---

## 5. Interaction with the deferred Phase 1 streaming features

This phase touches the **same decoder/scheduler seam** as the deferred Phase 1.3/1.4/1.5 items and the
1.6/1.7 robustness items. The interactions, explicitly:

- **1.3 Preload / prefetch (deferred; preload half).** *Shares machinery, does not conflict — and should
  be sequenced after.* Preload stages the **next track** into a second decoder instance during the
  current track's tail; windowing bounds the **current track's** forward buffer. They are orthogonal
  axes (next-track vs. current-track-window), but they compound the memory question: a naive preload of a
  second 1 GB mix would reintroduce the OOM this phase fixes. **Recommendation: land windowing first**,
  so that when preload arrives, the staged next-track decoder is *also* windowed by construction (it
  inherits the bounded scheduler). Windowing makes preload *safe for long tracks*; without it, preload of
  mixes is a memory hazard.
- **1.4 Crossfade (deferred).** Needs two simultaneous `PlaybackScheduler` instances briefly overlapping.
  Both would be windowed instances — the overlap doubles the *window* size momentarily, not the whole
  track. Windowing makes crossfade between two long mixes affordable. No reordering needed; 1.4 still
  gates on 1.3.
- **1.5 Gapless (deferred).** Sample-accurate hand-off of the next track's first buffer at the current
  track's last buffer. Windowing changes *which* buffers are retained but not the hand-off mechanism;
  the only care point is that the current track's **final** window must not be evicted before the gapless
  boundary is scheduled. A minor invariant for whoever builds 1.5, not a blocker. Note 1.5's existing
  WAV-only caveat stands.
- **1.6 Track-skip on error (deferred).** *Windowing enlarges the error surface — call this out.* Today
  a fetch failure happens at load (one fetch) or at a user seek (one fetch). Windowed refill issues
  **mid-stream** fetches the listener did not initiate; one of those can fail at byte 700 M of a 1 GB
  mix. So Phase 21 should ship with at least the *cheap* half of 1.6: a mid-stream refill failure must
  **surface a clear error and not wedge the player** (it must not leave playback "running" with a starved
  scheduler — mirror the `playFromPosition` end-of-buffer recovery already in `PlaybackScheduler`). The
  rich half (byte-scan to next valid frame) stays deferred. **Recommendation: fold the minimal refill-
  failure handling into Phase 21's acceptance criteria** (AC6) rather than leaving it entirely to 1.6 —
  it is created by this phase.
- **1.7 Safari compatibility (deferred).** Windowing adds no new Safari-specific surface beyond what the
  streaming path already has. The one adjacency: more frequent `AudioContext` activity during refill
  should be checked against the older-Safari `webkitAudioContext` quirks when 1.7 is addressed — note it,
  do not block on it.

---

## 6. Open questions for Daniel (genuine product decisions, not implementation detail)

These are policy calls with user-visible or resource trade-offs — flagged rather than decided here.

- **OQ1 — Window size policy.** What bounds the window — a **fixed byte/time budget** (e.g. "hold at
  most ~30 s decoded ahead + ~10 s behind"), or a **configurable memory budget** (e.g. "≤ N MB of
  decoded PCM") that derives the time window from the stream's byte rate? Recommend a **time-based
  forward window + small time-based back-retain** as the primary knob (intuitive, format-portable), with
  a hard **memory ceiling** as a secondary guard. The exact numbers are tunable post-landing; Daniel
  picks the *policy axis*. `[Daniel decision]`
- **OQ2 — Seek-back past the evicted window.** When the listener seeks back earlier than the retained
  tail, we must refetch (the audio is gone). Acceptable to take the same brief re-buffer the forward
  seek-beyond-buffer takes today? (Recommend yes — it is the symmetric case and listeners already accept
  it forward.) Or should back-retain be generous enough that this is rare? `[Daniel decision]`
- **OQ3 — Configurable total in-flight memory cap.** Should there be a single hard byte ceiling on total
  decoded audio held by the player (a safety net independent of the window-size policy), exposed as a
  config value? Recommend **yes, as a guard rail** even if the window policy is time-based — it is the
  backstop that makes "1 GB stream never OOMs" a guarantee rather than a tuning hope. `[Daniel
  decision]`
- **OQ4 — Apply windowing to all tracks, or only long ones?** A 3-minute Cut decoded whole is ~30–60 MB
  — harmless today. Windowing everything is simpler (one code path) but adds refill machinery to short
  tracks that never needed it. Recommend **window everything** (one path, C6-safe, and short tracks
  simply never hit a refill because they fit inside the forward window) — but Daniel may prefer a
  size threshold. `[Daniel decision]`
- **OQ5 — Is MSE (Direction C) the real destination? — RESOLVED: NO (Daniel, 2026-06-23).** **Do not
  adopt MSE. The bespoke Web Audio decode→schedule graph stays — it is bespoke by deliberate choice, a
  long-term commitment, not a stopgap.** Daniel's rationale: the player is intentionally a custom
  graph, not an HTML `<media>` element; the compressed-delivery move that *would* have made MSE
  tempting is being met instead by **Phase 18 (Opus low-data path)** feeding the **same bespoke graph**
  through the `IFormatDecoder` seam — so compressed delivery arrives *without* surrendering the graph.
  Consequence for this phase: Direction A (the hand-rolled sliding window) is the destination, not a
  placeholder; invest in it as permanent machinery. It will window both the WAV and the Opus path
  (the sequencing note at the top). Direction C is recorded as **considered and declined** per file
  convention; kept visible so a future reader sees the road not taken and why.
  `[RESOLVED — bespoke graph retained; MSE rejected]`

---

## 7. Acceptance criteria

- **AC1 (headline) — Bounded memory under a 1 GB stream.** Playing a 1 GB+ WAV mix start to finish, the
  browser tab's retained decoded-audio memory stays bounded to the configured window (not growing toward
  ~2 GB). Verifiable via browser memory tooling: peak decoded-audio footprint is independent of track
  length and tracks the window-size policy, not the file size.
- **AC2 — Playback-start latency at parity (C2).** First-audio latency for a track is unchanged from
  pre-windowing (within noise). Windowing does not introduce a fetch-then-play stall.
- **AC3 — Continuous playback, no starvation.** A long mix plays edge to edge with no audible gaps,
  underruns, or stalls under normal network conditions — the forward fill stays ahead of the playhead.
- **AC4 — Seek-back within the window is instant (UC3).** A short backward seek into retained audio
  produces no network request.
- **AC5 — Seek (forward, and back past the window) still works (UC2/UC4).** Both resolve via the
  existing Range path with the same behavior the listener sees today; the pre-seek region is evicted, not
  retained.
- **AC6 — A mid-stream refill failure degrades cleanly (the 1.6 adjacency).** A failed refill fetch
  surfaces a clear user-visible error and leaves the player in a recoverable state (not a wedged
  "playing" with a starved scheduler). It must not silently hang.
- **AC7 — The Mix visualizer is unaffected (C7).** With the lava visualizer running on a long mix, the
  visualizer renders identically (it reads the preprocessed datum, never the evicted buffers).
- **AC8 — Single-decoder concurrency invariant holds (C6).** Under rapid seek + refill activity, no
  interleaved `ProcessStreamingChunk` calls corrupt the single JS decoder (the existing drain/cancel
  discipline still governs every fetch).

---

## 8. Wave decomposition

Dependency shape: `21.1 → 21.2 → 21.3`, with `21.4` validating the whole. 21.1 is the cold-start
prerequisite and the load-bearing change; the rest layer on it.

- **21.1 — Partial eviction in `PlaybackScheduler` (cold-start; the load-bearing change).** Give the
  scheduler the ability to drop already-played buffers and keep its position/index bookkeeping correct
  against a buffer array that no longer begins at absolute time 0 (today `getCurrentPosition`,
  `playFromPosition`, and the scheduling loop all assume `buffers[0]` is the track start). This is the
  hardest correctness work in the phase — the time-anchor math must stay exact through eviction. No
  refill yet; with eviction alone and the forward read loop unchanged, this is provably memory-bounded
  for the *played* region. **Independent of the §6 open questions** — it can begin immediately; the
  window *sizes* (OQ1/OQ3) are parameters fed in later. Settled and cold-start.
- **21.2 — Back-pressure on the forward read loop (the bound on the *unplayed* region).** Make the C#
  `StreamAudioWithEarlyPlayback` loop stop calling `ReadAsync` when forward decoded lookahead exceeds the
  high-water mark, and resume below low-water. Together with 21.1, this bounds *both* the played and
  unplayed sides — the full memory guarantee (AC1). Must route resume/pause through the existing
  cancellation-safe single-loop discipline (C6). **Depends on 21.1** (eviction must exist so the drained
  region is reclaimed, not merely un-read).
- **21.3 — Seek-back-past-window refill (close the random-access case).** Wire UC4 — when a backward
  seek lands earlier than the retained tail, refetch via the existing seek-beyond-buffer Range path
  pointed at the earlier offset, and the minimal AC6 refill-failure handling. Mostly **reuse** of the
  landed seek path; the new work is the trigger (window-miss detection) and the clean-failure path.
  **Depends on 21.1 + 21.2** (needs the window boundaries they define).
- **21.4 — Validation pass against the 1 GB target (acceptance).** Exercise AC1–AC8 against a real 1 GB+
  mix: memory profiling (AC1), latency parity (AC2), edge-to-edge playback (AC3), the seek matrix
  (AC4/AC5), induced refill failure (AC6), visualizer-running (AC7), and rapid-seek concurrency (AC8).
  Largely test/measurement; any break is likely a tuning fix in the 21.1 anchor math or the 21.2
  water-marks. **Depends on 21.1–21.3.**

---

## 9. Cross-references (read before implementing)

- Root `CLAUDE.md` "Streaming-first audio playback" / `CONTEXT.md §3.5` — the seam this phase modifies;
  the §2 invariants here restate its contract. Both flag it as the most load-bearing path.
- `PLAN.md` Phase 4 (landed) / `COMPLETED.md` — the HTTP Range `bytes=X-` primitive this generalizes.
- `PLAN.md` Phase 1.3 / 1.4 / 1.5 / 1.6 / 1.7 — the deferred decoder/scheduler-seam features; §5 above
  reconciles each.
- `PLAN.md` Phase 9 — defines the `Mix` medium (single long track), the canonical 1 GB case.
- `PLAN.md` Phase 10 / `product-notes/phase-10-mix-visualizer-lava-reframe.md` /
  `product-notes/phase-12-waveform-visualizer-generalization.md` — establishes the preprocessed
  per-track high-res waveform datum; the basis for C7 (visualizer does not read live PCM).
- `DeepDrftPublic/Interop/audio/PlaybackScheduler.ts` — owns the unbounded `buffers: AudioBuffer[]`;
  21.1 lives here.
- `DeepDrftPublic/Interop/audio/StreamDecoder.ts` — `reinitializeForRangeContinuation`,
  `calculateByteOffset`; the refill substrate.
- `DeepDrftPublic.Client/Services/StreamingAudioPlayerService.cs` — the C# forward read loop
  (`StreamAudioWithEarlyPlayback`), the seek-beyond-buffer path (`SeekBeyondBuffer`), and the
  cancellation/drain discipline (C6); 21.2/21.3 live here.
- `DeepDrftPublic.Client/Clients/TrackMediaClient.cs` — the Range-capable media fetch reused by refill.
